JP2001063541A - Brake control device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To prevent noise generation and energy consumption at the time of preload in a brake control device capable of securing control responsiveness by performing preload before control is started. SOLUTION: When it is determined that automatic braking is necessary, a brake circuit c is cut off by a brake circuit opening / closing means h,
A brake control device having a control means n for operating the motor d to supply the brake fluid to the brake circuit c and performing automatic braking control for controlling the fluid pressure control means f to automatically generate a braking force, While the automatic braking control is executed when the output value from the braking determination means k exceeds the first threshold value, the second output value from the braking determination means k is lower than the first threshold value. When the preload control is performed between the threshold value and the first threshold value, a preload control for applying a preload upstream of the hydraulic pressure control means f is performed. When the preload control is performed, the automatic braking control is performed. The motor m is driven at a lower current value than when it is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a brake control device, and more particularly to a brake control device for generating a braking force on a desired wheel when the vehicle becomes unstable due to a so-called over-oversteer state or over-understeer state. Vehicle behavior control that generates a yaw moment in the direction that suppresses vehicle attitude change, slip prevention control that suppresses drive wheel slip by generating braking force on the drive wheel when slip occurs on the drive wheel, or set vehicle speed Automatically when the driver is not performing a braking operation, such as braking control that generates braking force when the vehicle needs to be decelerated in a so-called auto cruise state or automatic driving state by a control device The present invention relates to a brake control device that executes automatic braking control for generating a braking force.

[0002]

2. Description of the Related Art Conventionally, a brake control device for executing the above-mentioned automatic braking control has a pump as a hydraulic pressure source at the time of control, and at the time of executing the automatic braking control, the pump is driven to adjust the hydraulic pressure to a desired value. It is supplied to the wheel cylinder.

In such a configuration, it is necessary to shorten the time from when the execution of the control is determined to when the hydraulic pressure is actually supplied to the wheel cylinder as much as possible to ensure control responsiveness. In order to ensure control responsiveness as described above, a large pump having a high discharge pressure is required as a pump.

[0004] On the other hand, as a conventional technique which can ensure control responsiveness without using a large pump, a technique described in Japanese Patent Application Laid-Open No. 5-330417 is known. The technology described in this publication relates to a device for executing an acceleration slip control for suppressing an acceleration slip, and includes a road surface condition and a vehicle condition (a throttle opening, a throttle opening change amount, an engine speed, a speed change). If the value indicating these exceeds a predetermined value, the pump is driven prior to control to generate a preload, and then, when executing the acceleration slip control, the hydraulic pressure is immediately To ensure control responsiveness.

[0005]

However, in the above-mentioned prior art, an ON / OFF pump is used as the pump, and the pump motor is driven in a fully open state from the start of the preload. Therefore, if the time from the start of the preload to the execution of the acceleration slip control becomes long, the motor will perform the full-open drive for a long time, causing a problem of operating noise and an increase in battery current consumption. The problem of being an economy had arisen.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems. In a brake control apparatus capable of ensuring control responsiveness by performing preloading before starting control, noise generation at the time of preloading is provided. It also aims to prevent energy consumption.

[0007]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention provides a brake operation hydraulic pressure source for generating a brake pressure corresponding to a driver's brake operation, as shown in the claim correspondence diagram of FIG. a control circuit hydraulic pressure source e including a brake circuit c connecting the a and the wheel cylinders b of the respective wheels, a pump d for discharging brake fluid to the brake circuit c, and a motor m for driving the pump d. A hydraulic pressure control means f provided at a position closer to the wheel cylinder than the connection position of the discharge circuit of the pump d in the brake circuit c to control the wheel cylinder pressure; The brake circuit c is provided at a position closer to the brake operation fluid pressure source than the connection position.
A brake circuit opening / closing means h for switching off, a braking determination means k for determining the necessity of braking and a target hydraulic pressure of the wheel cylinder b based on the traveling state obtained from the traveling state detection means j, and When it is determined that the automatic braking is necessary based on the output of the brake circuit c, the brake circuit c is shut off by the brake circuit opening / closing means h, and the motor d of the control hydraulic pressure source e is operated to supply the brake fluid to the brake circuit c. And a control means n for controlling the operation of the hydraulic pressure control means f and automatically performing a braking control for automatically generating a braking force.
The control means n executes the automatic braking control when the output value from the braking determination means k exceeds a first threshold value, while the output value from the braking determination means k When the value is between the second threshold value lower than the threshold value and the first threshold value, the brake circuit c is cut off by the brake circuit opening / closing means h and the wheel cylinder side is controlled by the hydraulic pressure control means f. In a state where the motor m is operated in a state in which the preload control is performed upstream of the hydraulic pressure control means f, and when the preload control is performed, the preload control is lower than when the automatic braking control is performed. The motor m is driven by a current value.

According to a second aspect of the present invention, in the brake control device according to the first aspect, when the control means n executes the preload control, the control means n first sets the first preload control.
It is preferable to execute a start-up control for driving the motor at a duty ratio of, and then perform a preload maintaining control for driving the motor at a second duty ratio that is lower than the first duty ratio.

Further, as set forth in claim 3, in the brake control device according to claim 2, the control means n
Is configured to set the first duty ratio to a higher value as the first duty ratio in response to the battery voltage when the battery voltage is low as compared to when the battery voltage is high when executing the start control. Is preferred.

Alternatively, as set forth in claim 4, in the brake control device according to claim 2, the control means n sets a time for executing the start-up control in accordance with a battery voltage. It is preferable to set the time for executing the activation control to be longer than that in the case of high.

[0011]

According to the present invention, when executing the automatic braking control, first, when the output value from the braking determining means k exceeds the second threshold value, the preload control is executed, A preload is applied upstream of the pressure control means f. Therefore, after that, when the output value from the braking determination means k exceeds the first threshold value larger than the second threshold value,
Automatic braking control is started. At this time, pressure is applied to the brake circuit c in advance, so that automatic braking control can be executed with high responsiveness.

In executing the above preload control, in the present invention, the motor m is driven with a lower current value than when the automatic braking control is executed. Therefore, the noise generated in the pump d and the motor m and the energy consumption thereof are:
It will be lower than before.

According to the second aspect of the present invention, when executing the preload control, first, the start control for driving the motor d is executed, and then the second value which is lower than when executing the start control is executed. The preload maintaining control for driving the motor d at the duty ratio of is performed. Therefore, when starting the stopped motor d, a relatively large driving force is necessary. At this start, the driving control generates a driving force with a relatively large current by the first duty ratio. Thereafter, as the minimum current required for driving with the second duty ratio, noise and energy consumption can be suppressed as much as possible, and start-up can be performed reliably.

According to the third aspect of the present invention, when the start control is executed, the first duty ratio of the current supplied to the motor m is increased when the battery voltage is low when the battery voltage is low. In comparison, the first duty ratio is set to a higher value. Therefore, even when the battery voltage is low, it is possible to reliably start the battery with the minimum necessary current without running out of current.

[0015] On the other hand, in the invention according to the fourth aspect, when executing the start-up control, the execution time of the start-up control is set to be shorter when the battery voltage is low than when the start-up control is high. Set the execution time longer.
Therefore, when the battery voltage is low, the start-up current is output as long as possible, so that the start-up can be performed with a small amount of current. Therefore, the start-up can be performed with the minimum necessary current.

[0016]

Embodiments of the present invention will be described with reference to the drawings. First, FIG. 2 is a configuration diagram showing an embodiment of the present invention. In the figure, reference numerals 1 to 4 denote wheel speed sensors (running state detecting means) for detecting a rotation speed of a wheel, and each output a frequency signal corresponding to the rotation speed of the wheel using, for example, a pickup coil.

Numeral 5 is a steering angle sensor (running state detecting means) for detecting a steering angle of the steering wheel. For example, a frequency signal corresponding to the steering angular velocity is output by a phototransistor or the like, and this is integrated to process the steering angle. Perform detection.

Reference numeral 6 denotes a yaw speed sensor (running state detecting means).
Then, for example, a yaw speed is detected by receiving a Coriolis force from a tuning fork type strain gauge or the like.

Reference numeral 7 denotes a lateral acceleration (hereinafter referred to as lateral G) sensor (running state detecting means) which detects lateral acceleration by receiving a lateral force with a cantilever type strain gauge or the like.

Reference numeral 8 denotes a brake control device (brake determining means / control means) which reads the running state based on signals from the sensors 1 to 7 and operates the valves 13a to 13h of the brake hydraulic control actuator 13. By controlling,
The switching of a hydraulic supply source to be described later to the wheel cylinders 20 of each wheel and the control of the brake fluid pressure supplied to each wheel cylinder 20 are performed to control the braking force of each wheel. Similarly, it calculates the required engine torque based on the signals from the sensors 1 to 7 and transmits the required engine torque to the engine control device 9.

The brake hydraulic control actuator 13
Is for supplying brake fluid pressure to the wheel cylinders 20 of the respective wheels and controlling the brake fluid pressure, and is provided in the middle of brake pipes (brake circuits) 21, 22 and 23. That is, the brake pipes 21 to 23
A brake pipe 21 connecting the right front wheel and the left rear wheel wheel cylinders 20 and 20, a brake pipe 22 connecting the left front wheel and the right rear wheel cylinders 20 and 20, and a brake corresponding to the driver's pedal operation. Master cylinder 14 and each pipe 2 as a brake operating hydraulic pressure source for generating operating hydraulic pressure
And a brake pipe 23 connecting the first and second pipes 22. The brake hydraulic control actuator 13 is provided in the middle of the brake pipes 21 and 22 and controls hydraulic pressure supplied to each wheel cylinder 20 (hydraulic control valves) 13a to 13d. A control hydraulic power source 13 composed of a motor-driven hydraulic pump capable of arbitrarily increasing the pressure in response to a signal from the brake control device 8
i, a brake pipe (discharge circuit) 24 connecting the control hydraulic pressure source 13i and the brake pipe 23, and a hydraulic pressure supplied to the brake pipe 21 provided in the brake pipes 23 and 24. The shutoff valve 13e and the shutoff valve 13g for switching between the brake operating hydraulic pressure generated in the above and the hydraulic pressure of the control hydraulic power source 13i, and the shutoff valve 13f and the shutoff for performing the same switching for the brake pipe 22 The brake control device 8 is configured to control the pressure supply source for the wheel cylinder 20 individually and control the brake fluid pressure of each wheel cylinder 20 in accordance with a signal from the brake control device 8. The shut-off valves 13g, 13g
The image h corresponds to the brake circuit opening / closing means in the claims. Incidentally, although not shown in the present embodiment, the pump of the control hydraulic power source 13i is
A suction circuit is connected upstream of the shutoff valves 13g and 13h (on the side of the master cylinder 14), and the same function can be obtained by providing the shutoff valves 13e and 13f in this suction circuit.

Each of the shut-off valves 13e, 13f, 13
g and 13h are normally the shut-off valves 13 on the master cylinder 14 side so that the brake operation fluid pressure generated in the master cylinder 14 is transmitted to the brake pipes 21 and 22.
g and 13h are opened, and shutoff valves 13e and 13f on the hydraulic supply pump 13i side are closed.

With the configuration described above, various sensors 1 to
7, the traveling state of the vehicle is detected, and each wheel cylinder 2
In this embodiment, the automatic braking control is performed in the direction of suppressing the excessive oversteer state when the vehicle is in an excessive oversteer state. It is configured to execute a vehicle behavior control for generating a braking force on one of the left and right front wheels to generate a yaw moment. In the present embodiment,
The vehicle behavior control is configured to be performed based on the vehicle slip angle β, and the preload control is configured to be performed prior to the execution of the vehicle behavior control.
Hereinafter, this control will be described in detail for each embodiment.

(Embodiment 1) The control operation of the brake control device 8 of Embodiment 1 will be described with reference to the flowchart of FIG. Steps 101 to 105 are portions for detecting the running state. In step 101, the wheel speed V of each wheel is determined based on the input from each wheel speed sensor 1 to 4.
w, and in a subsequent step 102, the wheel speed Vw
The vehicle speed Vx is calculated based on
At 4, the yaw speed YAW and the lateral acceleration YG are calculated based on the inputs from the yaw speed sensor 6 and the lateral G sensor 7. Then, in step 105, β = ∫
The vehicle body slip angle β is determined based on the calculation of (YG / Vx-YAW) dt.

In the following step 106, it is determined whether or not the obtained vehicle body slip angle β has exceeded a first threshold value B1.
If β> B1, the routine proceeds to step 107, where the motor of the control hydraulic power source 13i is driven at a PWM duty ratio of 100%. Step 106 corresponds to a braking determination means in the claims. Further, at step 108, a start timer described later is cleared, and at step 109
, The shutoff valves 13g and 13h are shut off, and the shutoff valves 13e and 13f are opened to use the hydraulic pressure source as the control hydraulic pressure source 13i. By controlling the opening and closing of the hydraulic control valves 13a to 13d in the subsequent step 120, the behavior of the vehicle can be controlled by optimally controlling the hydraulic pressure of each wheel cylinder 20. The portion described as the region C in the drawings of steps 107 to 109 corresponds to a portion in which the automatic braking control in the claims is executed.

On the other hand, in step 110, which proceeds when the determination in step 106 is NO, it is determined whether or not the vehicle body slip angle β has exceeded a second threshold value B2 smaller than the first threshold value B1. If>β> B2, the routine proceeds to step 111, where it is determined whether or not the count of a start timer that counts when the control hydraulic power source 13i is started has reached a predetermined value Ta. Proceeding to step 112, the motor of the control hydraulic power source
M is driven at a duty ratio of 70%, and the following step 11
At 3, the count of the timer is incremented.
The reason for setting the duty ratio to 70% in step 112 is that a relatively large driving force is required at the time of starting. Incidentally, the portion described as region A in the figure is the portion where the start control is executed, and at this time, the shutoff valves 13e and 13f are opened, and the shutoff valves 13g and 13h are closed. . This is the same even in a region B described later.

If β> B2 in step 110 and the count of the timer exceeds the predetermined value Ta and the result of step 111 is YES, the program proceeds to step 11
4, the PW for the motor of the control hydraulic power source 13i
The duty ratio of M is set to 30%, and in the subsequent step 115, the activation timer is cleared. In the figure, B
The part displayed as the area is the part where the preload maintenance control is executed.

Steps 106 and 11
When the determination is 0 and NO, that is, when β ≦ B2, the start timer is cleared in step 116 and the driving of the motor of the control hydraulic power source 13i is stopped.

Next, an operation example of the first embodiment will be described with reference to a time chart of FIG. In the first embodiment, the slip angle β is viewed as indicating vehicle behavior. When the slip angle β changes as shown, first, the slip angle β exceeds the second threshold value B2 before reaching the first threshold value B1 at which the vehicle behavior control needs to be started. At this point, start control for starting the motor of the control hydraulic power source 13i at a PWM duty ratio of 70% is executed (steps 106 → 110 → 111 → 112). Here, by setting the duty ratio to 70%, it is possible to start up smoothly even at the time of start-up in which a relatively large driving force is required to move a stationary one.

After that, the slip angle β becomes equal to the first threshold value B.
If it is maintained between 1 and the second threshold value B2, after a predetermined time Ta has elapsed from the start of the motor, the start control is terminated, and the motor of the control hydraulic power source 13i is switched to the PWM duty ratio 30. % Is performed (steps 106 → 110 → 111 → 114).

As described above, the control hydraulic power source 13i performs the start-up control and the preload maintaining control shown in the area A and the area B in the drawing to thereby control the brake pipes 21 and 22.
At the time when the vehicle behavior control is started, that is, when the slip angle β exceeds the first threshold value B1,
The vehicle behavior control can be executed by supplying the hydraulic pressure to the wheel cylinder 20 immediately.

When the slip angle β exceeds the second threshold value B2 and then falls again below the second threshold value B2, the driving of the motor is stopped (steps 106 → 110 → 1).
16 flow).

As described above, in the present embodiment,
When the slip angle β exceeds the second threshold value B2 which is smaller than the first threshold value B1 for starting the vehicle behavior control and the preload control for applying the preload is performed, when the motor is started, it is assumed that the motor is stopped. Start-up control is performed to drive the motor at a 70% duty ratio so as to obtain a relatively large driving force to move the motor. This start-up control is performed until a predetermined time Ta elapses. Is a hydraulic pressure control valve 13a, b, c, to execute preload maintenance control for driving the motor at a 30% duty ratio.
The power consumption of the battery and the operating noise of the motor can be minimized while reliably applying a preload upstream of d.

(Other Embodiments) Other embodiments will be described below. In describing these other embodiments, configurations and processing steps common to the first embodiment are denoted by common reference numerals, and description thereof is omitted.

(Embodiment 2) FIG. 5 shows a flowchart for executing the control of Embodiment 2. The difference from the first embodiment is that step 212 following step 111
Is that the driving of the motor of the control hydraulic power source 13i is variably controlled according to the battery voltage.

FIG. 6 is a flow chart showing the flow of step 212. In step 212a, it is determined whether or not the battery voltage Vb is less than 8V. In this case, the process proceeds to step 212b.

At step 212b, the battery voltage Vb
Is determined to be greater than or equal to 12V, and if it is greater than 12V, the process proceeds to step 212c, and if less than 12V, the process proceeds to step 212d.

At step 212c, the motor is driven at a duty ratio of 60%, at step 212d, the motor is driven at a duty ratio of -10 * Vb + 180%, and at step 212e, the motor is driven at a duty ratio of 100%.

That is, in the second embodiment, in the area A, the motor is driven with the duty ratio characteristics as shown in FIG.

FIG. 8 shows an operation example of the second embodiment.
The difference from the first embodiment is that the duty ratio is variably controlled in a range from 60% to 100% in accordance with the battery voltage Vb in the region A in the figure in which the start control is executed.

Therefore, when the battery voltage is higher than 12 V and sufficiently high, the motor is driven at a duty ratio of 60% in order to obtain a large driving force when starting the motor. On the other hand, if the battery voltage is less than 8V and the voltage is not sufficient, the driving force is reduced. If the duty ratio is low, the motor may not be able to start up against the load. It is driven at a duty ratio of 100%. When the battery voltage Vb is 8
In the range from to 12 V, the driving force is changed in accordance with the battery voltage so that the minimum driving force required for starting is obtained.

With the above control, in the present embodiment, the motor can be reliably started with the minimum necessary current, and the generation of noise and energy consumption can be suppressed to the minimum.

(Embodiment 3) FIG. 9 shows a flowchart for executing the control of Embodiment 2. The difference from the first embodiment is that the battery voltage Vba is read in step 301 where the operation proceeds to YES in step 110, and that step 312 following step 111 is performed.
, The threshold value Ta of the measurement time of the activation timer, which is the time for executing the activation control, is variably controlled according to the battery voltage.

FIG. 10 is a flowchart showing the flow of step 312. In step 312a, it is determined whether or not the battery voltage Vba is less than 8V. If the battery voltage Vba is less than 8V, the process proceeds to step 312e. , Proceed to step 312b.

In step 312b, the battery voltage Vb
It is determined whether or not a is greater than 12v. If a is greater than 12V, the process proceeds to step 312c, and if a is 12V or less, the process proceeds to step 312d.

In step 312c, Ta = Ta2 is set. In step 312d, Ta =-((Ta1-Ta)
2) / 4) * Vb + (3Ta1 + 2Ta2) is set, and in step 312e, Ta = Ta1.

That is, in the third embodiment, in the region A, the motor is driven with timer characteristics as shown in FIG.

FIG. 12 shows an operation example of the third embodiment. The difference from the first embodiment is that the measurement of the start timer according to the battery voltage Vb in the region A where the control hydraulic power source 13i is started. The point is that the time is variably controlled.

Therefore, when the battery voltage is higher than 12 V and sufficiently high, the start-up control is executed in a short time by shortening the time of driving at the duty ratio of 70%, while
If the battery voltage is less than 8 V and is not sufficient, the driving force is reduced. Therefore, the time for driving at a duty ratio of 70% is lengthened, and the start is sufficiently executed even in this case. Also, when the battery voltage Vb is 8 to 12
Between V, the measurement time of the start-up timer is changed according to the battery voltage, and similarly, the preload is reliably performed with the minimum required noise and energy consumption.

Although the embodiment has been described with reference to the drawings, the configuration of the present invention is not limited to the configuration of this embodiment. For example, although the master cylinder is shown as a brake operation hydraulic pressure source, it is only necessary that any means that generates hydraulic pressure according to the braking operation of the driver be used. However, the present invention is not limited to this, and means for electrically detecting the braking operation of the driver and generating a hydraulic pressure according to the detected value may be used.

In the embodiment, an example in which the present invention is applied to vehicle behavior control has been described. However, the present invention can be applied to other automatic braking control. For example, when applied to drive wheel slip control,
The slip amount is compared with a threshold value, and when the slip amount exceeds a second threshold value, preload control is executed, and thereafter, when the slip amount exceeds the first threshold value, drive wheel slip control is executed. Also, when executing control to follow the preceding vehicle, the preload control and the automatic braking control are performed by comparing the threshold value with the distance between the preceding vehicle and the relative speed change rate with the preceding vehicle. I do.

[Brief description of the drawings]

FIG. 1 is a diagram corresponding to claims showing the present invention.

FIG. 2 is an overall view showing a brake device to which an embodiment of the present invention is applied.

FIG. 3 is a flowchart illustrating a control flow according to the first embodiment;

FIG. 4 is a time chart showing an operation example of the first embodiment.

FIG. 5 is a flowchart showing a control flow according to the second embodiment.

FIG. 6 is a flowchart showing a control flow of a main part of the second embodiment.

FIG. 7 is a duty characteristic diagram according to the second embodiment.

FIG. 8 is a time chart showing an operation example of the second embodiment.

FIG. 9 is a flowchart illustrating a control flow according to the second embodiment.

FIG. 10 is a flowchart showing a control flow of a main part of the second embodiment.

FIG. 11 is a duty characteristic diagram according to the second embodiment.

FIG. 12 is a time chart showing an operation example of the second embodiment.

[Explanation of symbols]

 a brake operation hydraulic pressure source b wheel cylinder c brake circuit d pump e control hydraulic pressure source f hydraulic pressure control means g discharge circuit h brake circuit opening / closing means j running state detecting means k braking determination means m motor n control means 1-4 Wheel speed sensor 5 Steering angle sensor 6 Yaw speed sensor 7 Lateral acceleration sensor 8 Brake control device 13a-d Hydraulic control valve 13e-h Shut-off valve 13i Hydraulic supply pump 14 Master cylinder 20 Wheel cylinder 21 Brake pipe 22 Brake pipe 23 Brake pipe 24 Brake piping

Claims (4)

[Claims] 1. A brake circuit that connects a brake operation fluid pressure source that generates a brake pressure according to a driver's brake operation and a wheel cylinder of each wheel, a pump that discharges brake fluid to the brake circuit, and the pump. A control hydraulic pressure source composed of a motor that drives a pump, a hydraulic pressure control unit that is provided at a position closer to the wheel cylinder than the connection position of the discharge circuit of the pump in the brake circuit, and is capable of controlling the wheel cylinder pressure, A brake circuit opening / closing means provided at a position closer to a brake operating fluid pressure source than a connection position of a discharge circuit of the pump in the brake circuit to switch communication / cutoff of the brake circuit; and a running state obtained from a running state detecting means. Braking determination means for determining the necessity of braking and the target hydraulic pressure of the wheel cylinder, and output from the braking determination means. Together when it is determined that requires automatic braking blocks the brake circuit the brake circuit closing means and to actuate the motor control hydraulic pressure source is supplied to the brake fluid to the brake circuit, based on,
A control unit that controls the operation of the hydraulic pressure control unit and performs automatic braking control that automatically generates a braking force.The brake control device, wherein the control unit outputs an output value from the braking determination unit. First
The automatic braking control is executed when the threshold value exceeds the threshold value, and the second threshold value and the first threshold value, the output values of which are lower than the first threshold value, are output from the braking determination means. When the pressure is between the threshold values, the brake circuit is opened and closed by the brake circuit opening / closing means, and the motor is operated while the wheel cylinder side is shut off by the hydraulic pressure control means to provide a preload control upstream of the hydraulic pressure control means. And a motor configured to drive the motor with a lower current value when performing the preload control than when performing the automatic braking control. 2. When executing the preload control, the control means firstly executes a start control for driving the motor at a first duty ratio, and thereafter executes a value lower than the first duty ratio. 2. The brake control device according to claim 1, wherein the preload maintaining control for driving the motor at the second duty ratio is performed. 3. The control unit according to claim 1, wherein the control unit sets the first duty ratio to be higher when the battery voltage is low than when the battery voltage is higher than when the battery voltage is high. The brake control device according to claim 2, wherein the brake control device is configured to be set to a value. 4. The control unit is configured to set a time for executing the start control to be longer when the battery voltage is low than when the battery voltage is high, in accordance with the battery voltage. The brake control device according to claim 2, wherein